CHIRAL RECOGNITION BETWEEN DISSYMMETRIC LN(DPA)3(3-) AND CO(III)-NUCLEOTIDE COMPLEXES IN AQUEOUS-SOLUTION - ENANTIOSELECTIVE LUMINESCENCE QUENCHING AS A PROBE OF INTERMOLECULAR CHIRAL DISCRIMINATION

Time-resolved chiroptical luminescence spectroscopy is used to investigate enantioselective excited-state quenching processes in solution samples that contain a racemic mixture of chiral luminophores and a small concentration of dissymmetric quencher molecules. The luminophores are Ln(dpa)3(3-) complexes (Ln = Eu3+ or Tb3+, dpa = a dipicolinate dianion) that have tris(terdentate) chelate structures of D3 symmetry with either left-handed (LAMBDA) or right-handed (DELTA) configurational chirality about the trigonal axis. The quenchers are 6-coordinate Co(III) complexes with the general stoichiometric formula Co(NH3)4(ndp) or Co(NH3)4(ntp), where ndp denotes a diphosphate nucleotide ligand and ntp denotes a triphosphate nucleotide ligand. In these complexes the nucleotide ligand is coordinated to Co(III) via bidentate chelation in which the donor moieties are oxygen atoms from adjacent phosphate groups (beta, alpha in ndp and gamma, beta in ntp nucleotides). The nucleotides used as ligands in Ln in this study are the following: adenosine diphosphate and triphosphate (adp and atp), guanosine diphosphate and triphosphate (gdp and gtp), inosine diphosphate and triphosphate (idp and itp), cytidine triphosphate (ctp), uridine triphosphate (utp), and deoxythymidine triphosphate (ttp). The Co(NH3)4(nucleotide) complexes have dissymmetric structures both within and external to the Co(NH3)4(phosphate) coordination unit, but under the conditions employed in this study, they exist as mixtures of diastereomers that are quasi-racemic with respect to chirality within the Co(NH3)4(phosphate) coordination units. All of the Co(NH3)4(ndp) and Co(NH3)4(ntp) complexes (denoted hereafter by CoNDP and CoNTP, respectively) perform as quenchers of Eu(dpa)3(3-) (Eu*) and Tb(dpa)3(3-) (Tb*) luminescence, and the quenching rate constants exhibit significant variations among the different systems. For Eu* quenching, the rate constants vary from 2.2 x 10(6) to 16.9 x 10(6) M-1 s-1, and for Tb* quenching, the rate constants vary from 4.3 x 10(6) to 14.4 x 10(6) M-1 s-1. Additionally, all of the CoNDP and CoNTP complexes exhibit enantiodifferential quenching of LAMBDA-Ln* vs DELTA-Ln* enantiomers (of the Eu(dpa)3(3-) and Tb(dpa)3(3-) luminophores), and the differential rate parameters associated with this quenching reveal striking variations in both the degree and sense of enantiomeric preference shown by the various quencher complexes. CoATP shows an enantiomeric preference that is the same as that shown by CoCTP, CoUTP, and CoTTP, but is opposite that of CoGTP, CoITP, CoGDP, CoIDP, and CoADP. Furthermore, whereas CoATP shows a 32% greater preference for one Tb* enantiomer over the other, CoADP shows only a 1% preference (for the opposite enantiomer). All of the CoNDP and CoNTP complexes have the same Co(NH3)4(phosphate) coordination unit, and the observed variations in the quenching properties (within either the CoNDP or CoNTP series) must be attributed to structural differences between their nucleoside base moieties. The quenching properties and their variations are discussed within the context of a model in which it is assumed that quenching occurs via Ln*-to-Co electronic energy-transfer processes in transient contact encounters between the luminophore and quencher species in solution.